Optical scanner with non-redundant overwriting

ABSTRACT

An optical scanning device is provided which comprises a laser array which emits laser beams including a number of beams (1, 2, . . . , n) writing a swath of rasters having a laser scanning section which, when an interlaced scanning period i, is set to a natural number between beams which are adjacent in a sub-scanning direction, scans the laser beams emitted from the laser array with the interlaced scanning period i. The laser scanning section can scan the laser beams such that the beam number n and the interlaced scanning period i are relatively prime natural numbers, and n&gt;i. In a first scan, data for raster lines (1, 2, . . . , n) can be selectively associated with a respective first exposure. At a second scan, data for raster lines (i+1, i+2, . . . , n) can be selectively associated with a respective second exposure and data for raster lines (n+1, n+2, . . . , n+i) can be selectively associated with a respective first exposure. The first respective exposure for raster lines (i+1, i+2, . . . , n) is not equal to the respective second exposure for raster lines (i+1, i+2, . . . , n).

BACKGROUND

The present disclosure relates to an optical scanning device, andparticularly relates to an optical scanning device which is employed ina digital copier, a laser beam printer or the like and which carries outsimultaneous writing with a plurality of laser beams. In order to reducebanding in prints produced by a raster output scanner (ROS) withmultiple beams, it is often necessary to overwrite the exposed profileto average out nonuniformity caused by xerographic nonlinearities andbeam power nonuniformity and misalignment. However, overwriting canwaste ROS optomechanical bandwidth and driver bandwidth that could beused for applications such as increased image quality, electronicregistration and microprinting. It is highly desirable to utilize thisROS bandwidth thereby enabling these applications.

Conventional overwriting presents a significant bandwidth challenge tothe ROS while not fully utilizing that bandwidth, for example, the imagepath provides no more information bits to a ROS that is overwriting eventhough it is operating at twice the bandwidth. It is desirable to reapas many benefits as possible from a high bandwidth, overwriting ROS.

SUMMARY OF THE DISCLOSURE

In one aspect of the present disclosure, an optical scanning device isprovided which comprises a laser array which emits laser beams includinga number of beams (1, 2, . . . , ) writing a swath of rasters having alaser scanning section which, when an interlaced scanning period i, isset to a natural number between beams which are adjacent in asub-scanning direction, scans the laser beams emitted from the laserarray with the interlaced scanning period i. The laser scanning sectioncan scan the laser beams such that the beam number n and the interlacedscanning period i are relatively prime natural numbers, and n>i. In afirst scan, data for raster lines (1, 2, . . . , n) can be selectivelyassociated with a respective first exposure. In a second scan, data forraster lines (i+1, i+2, . . . n) can be selectively associated with arespective second exposure and data for raster lines (n+1, n+2, . . . ,n+i) can be selectively associated with a respective first exposure. Therespective first exposure for raster lines (i+1, i+2, . . . n) is notequal to the respective second exposure for raster lines (i+1, i+2, . .. n).

In another aspect, an optical scanning device is provided and comprisesa laser array which emits laser beams including a number of beams (1, 2,. . . , n) writing a swath of rasters having a laser scanning sectionwhich, when an interlaced scanning period i, is set to a natural numberbetween beams which are adjacent in a sub-scanning direction, scans thelaser beams emitted from the laser array with the interlaced scanningperiod i. The laser scanning section can scan the laser beams such thatthe beam number n and the interlaced scanning period i are relativelyprime natural numbers, and n>i. In a first scan, data for raster lines(1, 2, . . . , i) can be selectively associated with a respective firstlaser power exposure of either zero or power P₁ [0, P₁] and data forraster lines (i+1, i+2, . . . n) can be selectively associated with arespective second laser power exposure [0, P₂]. In a second scan, datafor raster lines (i+1, i+2, . . . , n) can be selectively associatedwith a respective first laser power exposure [0, P₁] and data for rasterlines (n+1, n+2, . . . , n+i) can be selectively associated with arespective second laser power exposure [0, P₂]. The raster lines (i+1,i+2, . . . , n) can be selectively associated with a laser powerexposure selected from the group consisting of 0, P₁, P₂, and (P₁+P₂)after the first scan and the second scan.

In still another aspect, an electronic registration method inassociation with an optical scanner is provided which comprisesinputting data from an electronic image path to a raster output scanner(ROS) wherein the ROS can be a multi beam scanner and the data is afirst binary representation of pixel values. The method further includesdriving the raster output scanner in a first scan to write image signalsbased on the data wherein the signals include a swath of raster linesproducing spatial regions having a first exposure. The method furtherprovides for driving the ROS in a second scan and providingnon-redundant data by the image path to overwrite one or more of thepreviously written image signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing schematic structure of an image-formingapparatus relating to an embodiment of the present disclosure.

FIG. 2 is a diagram showing exposure profiles formed in a sub-scanningdirection by laser beams on a scanning surface, in a case of scanningfor writing using a conventional optical scanning device;

FIG. 3 is a diagram describing a conventional technique for writing byinterlaced scanning;

FIG. 4 is a schematic example of a halftone dot image written with extralevels around the periphery to produce finer gray steps in a thresholdreproduction curve (TRC); and,

FIG. 5 is a diagram showing an electronic registration example usingnon-redundant overwriting.

DETAILED DESCRIPTION

FIG. 1 is a diagram showing general structure of an image-formingapparatus 10 relating to an embodiment of the present disclosure. Theimage-forming apparatus 10 is covered by a casing 12.

An image-forming section 14 is provided inside the casing 12. Theimage-forming section 14 is provided with a cylindrical photoreceptor 16and an optical scanning device 18. The photoreceptor 16 rotates at aconstant speed in the direction of an arrow a shown in FIG. 1. Theoptical scanning device 18 directs light beams based on desired imagedata toward the photoreceptor 16 (in the direction of an arrow b inFIG. 1) while scanning.

A charging unit 20 is disposed in a peripheral surface vicinity of thephotoreceptor 16. The charging unit 20 charges the photoreceptor 16uniformly as a result of the photoreceptor 16 rotating in the directionof arrow a at a predetermined speed. The rotation of the polygon mirror52 causes beams b to scan along a path that is parallel to the axis ofthe photoreceptor 16. The intensities of the beams are modulated in animagewise fashion as they are scanned, selectively discharging thephotoreceptor 16 and thereby creating a latent image on thephotoreceptor 16.

A developing unit 22 which supplies toner to the photoreceptor 16 isdisposed facing the peripheral surface of the photoreceptor 16, at adownstream side in the direction of rotation of the photoreceptor 16from a position of irradiation by the light beam or beams from theoptical scanning device 18. The toner supplied from the developing unit22 adheres at portions which have been irradiated by the light beam fromthe optical scanning device 18. As a result, a toner image is formed onthe peripheral surface of the photoreceptor 16.

A transfer charger 24 is disposed facing the peripheral surface of thephotoreceptor 16, at a downstream side of the photoreceptor 16 in thedirection of rotation of the photoreceptor 16 from the position at whichthe developing unit 22 is disposed (at a position vertically below anaxial center of the photoreceptor 16). The transfer charger 24 transfersthe toner image formed on the peripheral surface of the photoreceptor 16to paper 30, which is guided between the photoreceptor 16 and thetransfer charger 24 from a paper tray 26 or a manual feed tray 28.

A cleaner 32 is disposed facing the photoreceptor 16, at a downstreamside in the rotation direction of the photoreceptor 16 from the positionat which the transfer charger 24 is disposed. Toner remaining at theperipheral surface of the photoreceptor 16 after transfer is removed bythe cleaner 32.

The paper 30 to which the toner image has been transferred is ejected inthe direction of an arrow c. A fixing unit 38, which is structured toinclude a pressure roller 34 and a heating roller 36, is disposed at adownstream side from the photoreceptor 16 in the direction of ejectionof the paper 30. At the fixing unit 38, the paper 30 to which the tonerimage has been transferred and which is being ejected is subjected topressure and heated, and the toner is fixed by melting. That is, a“fixing process” is performed at the fixing unit 38, and thepredetermined image is recorded on the paper 30. The fixing process isachieved, and the paper 30 on which the image has been recorded isejected to a discharge tray 40.

In one exemplary embodiment to practice, the optical scanning device 18can be provided with an array-form semiconductor laser (below referredto as a “laser array”) and a polygon mirror (a rotating multi-facedmirror). The polygon mirror can be formed in a regular polygonal shapeand provided with a plurality of reflection surfaces at side facesthereof, and can be rotated at high speed by an unillustrated motor. Thelaser array can be a vertical cavity surface emitting laser (VCSEL) inwhich a plurality of light emitting spots are arranged in twodimensions.

The VCSEL can include a multi-spot laser diode in which the plurality oflight emitting spots are arranged in two dimensions. In one arrangement,the multi-spot laser diode can have a total of thirty-six light emittingspots disposed two-dimensionally with predetermined spacings, six in amain scanning direction by six in a sub-scanning direction. In anotherarrangement, the multi-spot laser diode can have a total of thirty-twolight emitting spots disposed two-dimensionally with predeterminedspacings, eight in one scanning direction by four in another scanningdirection. These exemplary arrangements are for illustration purposesand are not intended to be limiting of the disclosure. Other opticalscanner reductions to practice are well known to those skilled in theart. For example, the light emitting device 50 can be a single edgeemitting laser diode having a multiplicity of emitters on a single chip,or a multiplicity of diodes. The scanning function, shown implemented bya rotating polygon mirror 52, could also be implemented by anoscillating galvanometer or a micro-electro-mechanical system.

FIG. 2 is a diagram showing an exposure profile which is formed in thesub-scanning direction by the laser beams on the scanning surface in acase of scanning writing using this optical scanning device. Here, anumber of laser beams n=36, and a case of progressive scanning is shown.That is, neighboring beams are scanned along respectively neighboringscanning lines of a raster image.

The optical scanning device carries out simultaneous scanning of 1st to36th scanning lines using a 1 st laser beam to a 36th laser beam at atime of a 1 st cycle of scanning of the beams (represented by a scannumber (j=1) in the drawing). Then the optical scanning device carriesout simultaneous scanning of 37th to 72nd scanning lines with the laserbeams for scan number (j=2). Thereafter, scanning of blocks of 36 linesin order of scan numbers (j=3), (j=4), . . . is similarly sequentiallycarried out in the same manner.

FIG. 3 is a diagram describing a conventional technique for writing withinterlaced scanning. This technique is described in, for example,Japanese Patent No. 3,237,452. Herein, a natural number i (greater thanor equal to 2), which has a spacing r between two neighboring beamsdivided by a scanning line spacing p, is defined as an “interlacedscanning period”. Note that i=1 for progressive scanning.

For example, if the scanning period i=(r/p)=3, that is, if the spacing7′ between two beams is specified to be 3p (p being the scanning linespacing), then for the scan number (j=1), a 1st scanning line is formedby a 1 st laser beam and a 4th scanning line is formed by a 2nd laserbeam. Then, when the scan number (j=2), a 3rd scanning line is formed bythe 1 st laser beam and a 6th scanning line is formed by the 2nd laserbeam. In the same way, for scan number (j=3) and onward, 5th, 7th, 9th,. . . scanning lines are formed by the first laser beam and 8th, 10th,12th, . . . scanning lines are synchronously formed by the second laserbeam.

Note that when interlaced scanning is carried out by a multi-beamexposure device as described above, it is sufficient that the number nof laser beams and the interlaced scanning period i are relatively primenatural numbers. That is, the number of laser beams n and the interlacedscanning period i are not divisible by a common integer other than 1.

Thus, if an interlaced scanning period defined by a natural number i(greater than or equal to 2), which is a quotient when a spacing rbetween adjacent beams on the scanned surface is divided by the scanningline spacings, is set to i, then, to prevent the problem of imagedefects due to reciprocity law failures it is necessary to carry outinterlaced scanning in a case such that n and p satisfy the followingequation: [1/n*p] is less than or equal to 3 cycle/mm.

It is to be appreciated that the number of lasers n and the interlacedscanning period i can be set up such that when the number of lasers n is16 or 32, interlaced scanning can be set up with i=3, 5, 7, 9, 11, 13,15, 17, 19 . . . , wherein a relationship between the number of beams nand the interlaced scanning period i satisfy the condition i<n.

Embodiments of the present disclosure will be described in detail withreference to FIGS. 4 and 5. The present disclosure provides animager/image path architecture that enables non-redundant overwriting.That is, a second exposure that is overwriting some number of rasterscan be different than a first exposure. The imager provides theoverwriting, while the image path provides the data for non-redundantwriting. There can be at least two options in the non-redundantoverwriting. Option (1) provides consistent power yielding threeexposure levels for a 2-pass system: 0 on, 1 on, or 2 on. This isanalogous to 3-level amplitude modulation, only without the laser poweractually being modulated at different levels. Option (2) providesdifferent laser power which can be used in subsequent passes (P₁, P₂)yielding 4 levels for a 2-pass system: [0, P₁, P₂, P₁+P₂]. Non-redundantoverwriting provides some interlaces that cannot be achieved by simplyincreasing the raster resolution. Most applications, benefits, andfeatures that could be enabled by high addressability are enabled by thepresent disclosure. One application includes electronic registrationbecause it uses high bandwidth in the image path at the very last stageprior to driving the ROS. The present disclosure can be utilized at theVCSEL ROS.

In one exemplary arrangement, a printing system can utilize a laserscanner as a ROS and can nominally write at 2400 spi (spots per inch).The description that follows for this exemplary arrangement is forillustration purposes and is not intended to be limiting of thedisclosure.

The present disclosure provides for an apparatus and a method. Thegeneral method can utilize several steps, to be described hereinafter.Data from an electronic image path can be input to a ROS. The commonform of data presented to the ROS is a binary [0, 1] representation ofpixel values, but image data at other quantization are within the scopeof the present invention. The data may be stored in a scan line bufferor scan line buffers and read out in a manner clocked with ROS writingtiming.

The data drives the ROS to write image signals such as a swath of rasterlines as produced by a multi-beam ROS. The data drive the ROS to producespatial regions having a first exposure. A typical scenario would be thewriting of a swath of beams in a multiple beam ROS, such as, forexample, a 32 beam VCSEL ROS. The beams can be spaced for the interlaceand magnification dictated by given design parameters. For instance, thebeams can be spaced at 2400 spi for writing with an interlace of 1.

Non-redundant data can be provided by the image path to drive the ROS tooverwrite at least some of the previously imaged signals. Additionaldata is provided to the ROS from the image path to overwrite one or moreof the previously written raster lines. One aspect of the presentdisclosure provides for additional data that can be different than thepreviously supplied data. In the case of a binary example, whenoverwriting a raster line, the data of the second writing may be 0 at alocation where the data of the first writing was 1, or vice versa.

In one exemplary arrangement, in a first scan, data for raster lines 1to 32 can be supplied to a 32 beam VCSEL ROS that writes a swath of 32rasters at 2400 raster/inch to produce a respective first exposure forraster lines 1 to 32. The data can be binary thereby producing spatialregions associated with “on” states and “off” states.

In a second scan, data for raster lines 17 to 48 can be supplied to theVCSEL ROS. The image plane is moved relative to the ROS so that thelower 16 rasters of the first swath (17 to 32) receive a respectivesecond exposure and rasters 33 to 48 receive a respective firstexposure. Data for the respective second exposure of rasters 17 to 32may be different that data for the respective first exposure of rasters17 to 32.

In a third scan, data for raster lines 33 to 64 can be supplied to theVCSEL ROS. The image plane is moved relative to the ROS so that thelower 16 rasters of the first swath (33 to 48) receive a respectivesecond exposure and rasters 49 to 64 receive a respective firstexposure. Data for the respective second exposure of rasters 33 to 48may be different that data for the respective first exposure of rasters33 to 48.

The present disclosure provides an optical scanning device which cancomprise a laser array which emits laser beams including a number ofbeams (1, 2, . . . , n) writing a swath of rasters having a laserscanning section which, when an interlaced scanning period i, is set toa natural number between beams which are adjacent in a sub-scanningdirection, scans the laser beams emitted from the laser array with theinterlaced scanning period i. The laser scanning section can scan thelaser beams such that the beam number n and the interlaced scanningperiod i are relatively prime natural numbers, and n>i. In a first scan,data for raster lines (1, 2, . . . , n) can be selectively associatedwith a respective first exposure and wherein at a second scan, data forraster lines (i+1, i+2, . . . , i) can be selectively associated with arespective second exposure and data for raster lines (n+1, n+2, . . .n+i) can be selectively associated with a respective first exposure. Thefirst exposure for raster lines (i+1, i+2, . . . , n) is not equal tothe respective second exposure for raster lines (i+1, i+2, . . . , n).And wherein at a third scan, data for raster lines (n+1, n+2, . . . ,n+i) can receive a respective second exposure and data for raster lines(n+i+1, n+i+2, . . . , n+n) can receive a respective first exposure. Theoptical scanning device can comprise data that is a binaryrepresentation of pixel values thereby producing spatial regionsassociated with an on state and an off state.

In another exemplary embodiment, a ROS can be driven to write a halftonedot with gray edges for more levels and driver robustness. Whenoverwriting is employed the laser driver can be required to modulate at2× the speed of a ROS that does not employ overwriting. Hence the drivermay fail to deliver the required modulation at the high speeds. A singlepixel on or off may fail to form due the unusual high speeds incurredwhen overwriting.

To get around this potential driver problem while obtaining benefits ofa multi-level halftoning system, the edge pixels can be turned on or offwith 2 or more in a row in a raster line and have the second pass(layer) employ pixels in the opposite state. This will allow formationof halftone dots 100 with finely controlled incremental gray level stepsizes so that smooth transitions can be achieved in the gray level ofthe halftone dot. An example of a halftone dot written with extra levelsaround the periphery is shown in FIG. 4. Similar gray edge (1-layer)writing could be used to finely position edges of other imagestructures, such as line art.

In still another exemplary arrangement, a ROS can be driven to write ahigher number of gray levels at any pixel. For example, assume a firstexposure to a spatial region could be one of [0, P₁] and second exposureto that region could be one of [0, P₂], where the P's denote the laserpower used for that region. A given region could then receive a combinedexposure of either 0, P₁, P₂, or P₁+P₂. Hence, if P₁ does not equal P₂ a4-level system can be enabled from two binary sets of data.

If 2P₁ is approximately equal to P₂ and P₁+P₂ equals 1 (full exposure),then [0, P₁, P₂, or P₁+P₂] equals [0, ⅓, ⅔, 1] and the system possessesequal steps of quantization.

A simple method of enabling additional levels can be understood by usingthe 32 beam system, wherein 16 beams of the swath, say the first 16, canbe written with [0, P₁] and the other 16 beams can be written with [0,P₂]. Other 2-power exposure schemes are also possible. This method canbe generalized to a 2^(n) exposure scheme for 17 levels of overwriting.For example, a 3-level power exposure scheme (n=3) would result in 8possible exposures for a given region [i.e. 0, P₁, P₂, P₃, P₁+P₂, P₁+P₃,P₂+P₃, P₁+P₂+P₃].

The optical scanning device provides a laser array which emits laserbeams including a number of beams (1, 2, . . . , n) writing a swath ofrasters having a laser scanning section which, when an interlacedscanning period i, is set to a natural number between beams which areadjacent in a sub-scanning direction, scans the laser beams emitted fromthe laser array with the interlaced scanning period i. The laserscanning section can scan the laser beams such that the beam number nand the interlaced scanning period i are relatively prime naturalnumbers, and n>i. In a first scan, data for raster lines (1, 2, . . . ,i) can be selectively associated with a respective first laser powerexposure of [0, P₁] and data for raster lines (i+1, i+2, . . . n) can beselectively associated with a respective second laser power exposure [0,P₂]. In a second scan, data for raster lines (i+1, i+2, . . . , n) canbe selectively associated with a respective first laser power exposure[0, P₁] and data for raster lines (n+1, n+2, . . . , n+i) can beselectively associated with a respective second laser power exposure [0,P₂]. The raster lines (i+1, i+2, . . . , n) can be selectivelyassociated with a laser power exposure selected from the groupconsisting of 0, P₁, P₂, and (P₁+P₂) after the first scan and the secondscan. The optical scanning device can comprise a first laser powerexposure P₁ and a second laser power exposure P₂, wherein the firstlaser power exposure is not equal to the second laser power exposure.The data can be a binary representation of pixel values therebyproducing spatial regions selected from the group consisting of noexposure in either pass, exposure in one pass using power P₁, exposurein one pass using power P₂, and exposure in two passes using power[P₁+P₂]. In one arrangement, two times P₁ can be substantially equal toP₂. P₁ plus P₂ can equal full exposure. In another arrangement, P₁ is ⅓of full exposure and P₂ is ⅔ of full exposure.

An electronic registration system, having multiple gray levels, canaccount for system distortion as will be described hereinafter. Oneobjective is to write an object in a desired position, given that therasters can be slanted due to a distortion such as skew or bow.Referring now to FIG. 5, wherein it is shown that an object is intendedto be written with horizontal edges, but the raster are slanted due tosome system distortion. FIG. 5 shows that the use of single writing(single layer) and overwriting with different laser powers to achievedesired edge positions. Other distortions (bow, skew, scan nonlinearity,magnification, displacement) could also be compensated using thismethod. This application does not require high bandwidth for theentirety of the image path. In one reduction to practice, the power andoverwrite decisions can be made in an electronic registration modulethat is at the end of the image path, just prior to the ROS. The opticalscanning device can include a selective power exposure which can beequal to 2 to the power of the (number of laser power levels). It is tobe appreciated that if K passes of overwriting occurs, then a rasterline can be overwritten K times in subsequent passes and each pass coulduse a common power level or a unique power level.

In some instances the benefits described above can be obtained bywriting at a higher raster resolution versus non-redundant overwriting.For instance, writing at 4800 rasters/inch yields many of the advantagesof overwriting with non-redundant data at 2400 rasters/inch. But, theoverwriting scheme allows additional interlace schemes that are notobtainable at 4800 rasters/inch. Table 1 provides some examples thatshow that, for the same magnification and bandwidth, the overwritingscheme enables different interlace options for different numbers ofbeams compared to the 4800 spi case.

TABLE 1 Resolution Beam spacing # of beams Interlace Overwrite(rasters/inch) (microns) 2 3 4 5 6 7 8 30 31 32 1 y 2400 10.6 x x x x xx 2 n 4800 10.6 x x x x

The electronic registration method, in association with an opticalscanner, provides inputting data from an electronic image path to a ROSwherein the ROS can be a multi-beam scanner and the data is a firstbinary representation of pixel values. The method further includesdriving the raster output scanner in a first scan to write image signalsbased on the data wherein the signals include a swath of raster linesproducing spatial regions having a first exposure. The method furtherprovides for driving the ROS in a second scan and providingnon-redundant data by the image path to overwrite one or more of thepreviously written image signals. The non-redundant data can be a secondbinary representation of pixel values different from the first binaryrepresentation of pixel values. The first and second binaryrepresentation of pixel values can produce spatial regions selected fromthe group consisting of no exposure in either scan, exposure in one scanusing a first power, exposure in one scan using a second power, andexposure in two scans using both the first power and the second power.The exemplary embodiments have been described with reference to thespecific embodiments. Obviously, modifications and alterations willoccur to others upon reading and understanding the preceding detaileddescription. It is intended that the exemplary embodiments be construedas including all such modifications and alterations insofar as they comewithin the scope of the appended claims or the equivalents thereof.

1. An optical scanning device comprising: a laser array which emitslaser beams including a number of beams (1, 2, . . . , n) writing aswath of rasters having a laser scanning section which, when aninterlaced scanning period i, is set to a natural number between beamswhich are adjacent in a sub-scanning direction, scans the laser beamsemitted from the laser array with the interlaced scanning period i, thelaser scanning section scans the laser beams such that the beam number nand the interlaced scanning period i are relatively prime naturalnumbers, and n>i; wherein in a first scan, data for raster lines (1, 2,. . . , n) are selectively associated with a respective first exposure;at a second scan, data for raster lines (i+1, i+2, . . . n) areselectively associated with a respective second exposure and data forraster lines (n+1, n+2, . . . , n+i) are selectively associated with arespective first exposure; and, said respective first exposure forraster lines (i+1, i+2, . . . , n) is not equal to said respectivesecond exposure for raster lines (i+1, i+2, . . . , n).
 2. The opticalscanning device of claim 1, wherein said data is a binary representationof pixel values thereby producing spatial regions associated with an onstate and an off state.
 3. The optical scanning device of claim 1,wherein at a third scan, data for raster lines (n+1, n+2, . . . , n+i)are selectively associated with a respective second exposure and datafor raster lines (n+i+1, n+i+2, . . . , n+n) are selectively associatedwith a respective first exposure.
 4. The optical scanning device ofclaim 3, wherein at said third scan said respective first exposure forraster lines (n+i+1, n+i+2, . . . , n+n) is not equal to said respectivesecond exposure for raster lines (n+1, n+2, . . . , n+i).
 5. The opticalscanning device of claim 4, wherein said data is a binary representationof pixel values thereby producing spatial regions associated with an onstate and an off state.
 6. An optical scanning device comprising: alaser array which emits laser beams including a number of beams (1, 2, .. . , n) writing a swath of rasters having a laser scanning sectionwhich, when an interlaced scanning period i, is set to a natural numberbetween beams which are adjacent in a sub-scanning direction, scans thelaser beams emitted from the laser array with the interlaced scanningperiod i, the laser scanning section scans the laser beams such that thebeam number n and the interlaced scanning period i are relatively primenatural numbers, and n>i; in a first scan, data for raster lines (1, 2,. . . , i) are selectively associated with a respective first laserpower exposure of zero and power level P₁ [0, P₁] and data for rasterlines (i+1, i+2, . . . , n) are selectively associated with a respectivesecond laser power exposure [0, P₂]; in a second scan, data for rasterlines (i+1, i+2, . . . , n) are selectively associated with a respectivefirst laser power exposure [0, P₁] and data for raster lines (n+1, n+2,. . . , n+i) are selectively associated with a respective second laserpower exposure [0, P₂]; and, said raster lines (i+1, i+2, . . . , n)selectively associated with a laser power exposure selected from thegroup consisting of 0, P₁, P₂, and (P₁+P₂) after said first scan andsaid second scan.
 7. The optical scanning device of claim 6, whereinsaid first laser power exposure P₁ is not equal to said second laserpower exposure P₂.
 8. The optical scanning device of claim 7, whereinsaid data is a binary representation of pixel values thereby producingspatial regions selected from the group consisting of no exposure ineither pass, exposure in one pass using power P₁, exposure in one passusing power P₂, and exposure in two passes using power P₁ and P₂.
 9. Theoptical scanning device of claim 6, wherein two times P₁ issubstantially equal to P₂.
 10. The optical scanning device of claim 6,wherein P₁ plus P₂ equals full exposure.
 11. The optical scanning deviceof claim 10, wherein P₁ is ⅓ of full exposure and P₂ is ⅔ of fullexposure.
 12. The optical scanning device of claim 8, wherein the numberof selective power exposures is equal to 2 to the power of the (numberof laser power levels).
 13. The optical scanning device of claim 6,wherein said data is a binary representation of pixel values therebyproducing spatial regions selected from the group consisting of noexposure in either pass, exposure in one pass using power P₁, exposurein one pass using power P₂, and exposure in x passes using powers (P₁,P₂, . . . , P_(x)).